1. Field of the Invention
The present invention relates to a friction spot joint structure in which a plurality of plate members are overlapped and joined by plastic-flowing the plate members.
2. Description of the Prior Art
It has been known conventionally that light metals, particularly aluminum and the like are unsuitable for fused joint such as are welding, compared with steels, because they are more conductive and transfer more heat than steels.
Taking the above into consideration, friction spot joint structure has been known in which plate members are point joined by fusing a part thereof by friction, for example, as disclosed in Japanese Patent Application Laid Open Publication No. 2002-292479A. In detail, in a friction spot joint apparatus including: a rotary tool having a pin portion at its tip end and a shoulder portion of a diameter larger than that of the pin portion at the base end of the pin portion; and a receiving member arranged so as to face the rotary tool in the axial direction of the rotation axis, first and second plate members overlapped with each other are interposed between the rotary tool and the receiving member, the pin portion is pressed into the first plate member and the second plate member, while rotating the rotary tool, and the shoulder portion is pressed against the first plate member in the axial direction by, whereby the plate members are point joined.
In the conventional friction spot joint structure, the pressure and number of rotation of the rotary tool and the joining period must be adjusted precisely. However, the adjustment is difficult because the relationship between the joint quality and such joining conditions is unclear. Hence, variation of the joint quality becomes large. Further, the point joint is performed by fusing plate materials by friction, which requires a considerable time period.